Export Problems: fine playback on Win, audio/video sync issues on Macs

Hi all,

fairly new to Premiere and video editing in general, but I've produced a few videos of virtual band performances during lockdown with minimal problems.

Having just completed the latest one a weird issue has surfaced - the exported file plays fine on my ( windows ) laptop, and when uploaded to Facebook it streams to my laptop with no playback issues.

But on mobile devices, and seemingly on Apple computers, there are significant video / audio sync issues, on playback, both when streaming or playing the original file. 
I've sent the file to a number of people to confirm - on a windows system they all got perfect playback, on Apple systems or mobile devices the sync was messed up, whether streaming or playing from the drive.

It's a fairly simple edit using only cuts and fades, and all the video has been handbraked to a constant 30fps before editing.

Tearing my hair out looking for a solution, so any help would be greatly appreciated!

[Moderator note: title edited for clarity.]